1. Field of the Invention
The present invention relates to a video signal scrambling system and, more particularly, to a video signal scrambling system for subscription broadcast or cablecast television systems which can scramble video signals without adversely effecting pictures received at a subscriber's terminal.
2. Description of the Prior Art
In a broadcast television (TV) or cablecast television (CATV) system, a subscription television system or a pay program system has been developing, in which a subscriber subscribes to a broadcast or cablecast station (hereafter referred as TV station) for a reception of programs. In the pay program system, a program is scrambled at the TV station and is transmitted to a subscriber's terminal. A descramble circuit is provided in the subscriber's terminal.
In the TV or CATV system based on the pay program system, it is necessary to scramble pay program signals in order to prohibit non-subscribers from receiving and enjoying the pay program. In this case, a key data to descramble the scrambled pay program is used in the subscriber's terminal.
In addition, it is necessary to prevent the pay programs received at the subscriber's terminal from being adversely effected by the scrambling operation.
An example of conventional video picture signal scrambling systems is described in this U.S. Pat. No. 4,308,556 issued on Dec. 29, 1981. In the scrambling system, programs are scrambled by reversing the brightness level of a video picture signal on a horizontal cycle when the average brightness level of the picture signal changes. In this system, a reversion of the picture signal is performed only when the average brightness level of the pictures has changed by a predetermined amount. Then, the scrambled program is transmitted to the subscriber's terminal, together with key data showing the locations of the level reversed pictures. The level reversed picture is then restored in the subscriber's terminal according to the key data.
In the conventional video picture signal scrambling system, flicker of the picture presented on a display, e.g., a cathode ray tube, is reduced to some degree. However, often the restoration of the level reversed picture takes place with reference to another level, different from the predetermined level used at the TV station. In that case, the brightness of the restored picture shifts from the brightness of previous and subsequent pictures. Thus, the flicker becomes more pronounced. Therefore, the conventional system still has a drawback in that the flicker is not sufficiently suppressed.
The conventional video picture signal scrambling system also has another drawback in that the channel of a scrambled program causes a cross modulation distortion to other channels. As well known, every channel causes the cross modulation distortion to other channels, in proportion to the square of the amplitude of its carrier wave. In the conventional video picture signal scrambling system, the carrier wave of the scrambled program channel can easily become excessively modualted in amplitude by the level reversed picture signal.